Multi-axial spinal fixation system

ABSTRACT

A spinal fixation system includes an elongated rod sized to extend between at least two vertebrae and a number of anchor devices for anchoring the rod to the spine. The anchor devices include a bone engaging fastener having a head defining a spherical socket. A ball insert is configured to be placed within the socket and rotated so that a spherical surface of the ball insert is juxtaposed with the spherical surface of the socket. The insert has a first dimension less than the socket opening and a second dimension that upon rotation causes the insert to be captively retained in the socket. The anchor device further includes a yoke defining a yoke channel for receiving the rod and including a stem that is engaged to the ball insert when the insert is captured within the socket. A sleeve is disposed between the yoke channel and the fastener head for supporting the rod thereon. A set screw is threaded into the yoke channel for clamping the rod against the sleeve. The set screw further operates to draw the ball insert into clamped engagement within the socket.

BACKGROUND

The present invention relates to spinal fixation systems andparticularly to an anchor device that incorporates multi-axial fixationto the spine.

Several techniques and systems have been developed for correcting andstabilizing injuries to or malformation of the spine. In one type ofsystem, an elongated member such as a bendable rod is disposedlongitudinally along a length of the spine, spanning two or morevertebral levels. In certain applications, the rod is bent to correspondto the normal curvature of the spine in the particular region beinginstrumented, such as the normal kyphotic curvature of the thoracicregion or the lordotic curvature of the lumbar region. In accordancewith such a system, the rod is engaged to various vertebrae along alength of the spinal column by way of a number of anchor devices thatutilize a variety of fixation elements configured to engage specificportions of the vertebra and other bones. For instance, one suchfixation element is a hook that is configured to engage the laminae ofthe vertebra. Another very prevalent fixation element is a screw thatcan be threaded into various parts of the vertebrae or other bones.

Early rod-type spinal fixation systems incorporated anchor devices thatpermitted very limited relative orientations of the rod relative to thefixation element. As these system evolved, various degrees of freedom ofrelative orientation were integrated into the system. For instance, inone system a bone screw may be engaged to the spinal rod at a range ofplanar angles. This so-called variable angle screw allows pivoting ofthe bone screw in a single plane parallel to the plane of the spinalrod. One goal achieved by the variable angle screw is that the surgeoncan apply vertebral fixation elements to the spine in more appropriateanatomic positions.

Another rod-type fixation system utilizes fixation elements having abody that defines a slot within which the spinal rod is received. Theslot includes a threaded bore into which a threaded plug is engaged tosecure the rod within the body of the fixation element. One benefit ofthis type of fixation element is that the fixation element may bepositioned directly beneath the elongated rod, thereby reducing theoverall bulkiness of the implant construct and minimizing trauma to thesurrounding tissue.

On the other hand, these so-called “open back” fixation elements arecapable only of pivoting about the spinal rod to achieve variableangular positions relative to the rod. While this limited range ofrelative angular positioning is acceptable for many spinal pathologies,many other cases require more creative orientation of a bone fastenerrelative to a spinal rod. Certain aspects of this problem are addressedby some prior multi-axial or poly-axial screws that are capable ofvarious three-dimensional orientations with respect to the spinal rod.One type of poly-axial screw design, shown in U.S. Pat. No. 6,537,276 toMetz-Stavenhagen et al., includes a spherical projection on the top ofthe bone screw. An internally threaded receiver member pivotallysupports the bone screw and a spinal rod on top of the sphericalprojection. An inner set screw is tightened into the receiver member topress the spinal rod against the spherical projection to accommodatevarious angular orientations of the bone screw relative to the rod. Asimilar multi-axial screw is disclosed in U.S. Pat. No. 5,466,237 toByrd et al., except an outer nut is provided to secure the rod againstthe head of the bone screw.

In another approach shown in U.S. Pat. No. 4,946,458 to Harms et al., aspherical headed bone screw is supported within separate halves of areceiver member. The bottom of the halves are held together by aretaining ring. The top of the receiver halves are compressed about thebone screw by nuts threaded onto a threaded spinal rod. One detriment ofthis system is that the spinal rod must be threaded in order to acceptthe compression nuts, which has a tendency to weaken the spinal rod inthe face of severe spinal loads. Harms et al. also describes in U.S.Pat. No. 5,207,678 another multi-axial pedicle screw wherein acompression member is provided between the rod and the head of the screwto exert a force on the screw head to lock the screw against the innerspherical surface of the receiver member.

Yet another approach is illustrated in U.S. Pat. No. 5,797,911 toSherman et al., in which a U-shaped holder is provided that receives abone fastener topped with a crown member. The holder accommodates a rodin a channel above the crown member and a compression member above therod. The compression member presses on the rod and crown member to lockthe fastener against the holder in any of a number of angles in threedimensions with respect to the rod. Another system shown in U.S. Pat.No. 5,733,285 to Errico et al., includes a holder having a tapered andcolleted portion into which a bone fastener head is inserted. A sleeveis provided that translates down around the colleted portion to crushlock the colleted portion around the head of the bone fastener. Thisapparatus is bulky and difficult to manipulate given the externalsliding locking mechanism. It is further dependent on the fit of theexternal sleeve and the relative strength of the collet and its bendingand crushing portions for secure locking of the bone fastener head.

There is thus a need for a multi-axial or poly-axial fastener for usewith a spinal fixation system that is simple to construct yet strongenough to withstand harsh spinal loads. This need should also befulfilled by an anchor device that avoids the bulkiness of prior systemsbut can still achieve a simple and easy fixation of the spinal rod tothe bone fastener mounted within the spine.

SUMMARY OF THE INVENTION

The present invention contemplates a spinal fixation system thatincorporates multi-axial fixation characteristics in a low-profile, easyto construct anchor device. The system includes an elongated member,such as a spinal rod, that extends between spinal segments. A series ofanchor devices anchor the rod to the spinal segments, with at least someof the anchor devices providing multi-axial fixation. In one embodimentof the invention, the multi-axial anchor device includes a bone engagingfastener that is adapted to engage a portion of the spine. In onespecific embodiment, the fastener is a bone screw adapted to be threadedinto the pedicle of a vertebra.

The head of the bone engaging fastener in such specific embodiment isprovided with a spherical socket facing the spinal rod. A ball insertelement is provided that incorporates a spherical surface for variableangular interface with the socket. The ball insert is further configuredso that the insert may be introduced into the socket and then rotatedwithin the socket so that the spherical surface is juxtaposed to thesocket for captive retention therein.

In one configuration, the socket has an interior diameter and a smallerdiametrical opening communicating therewith. The ball insert isconfigured as a truncated sphere having a spherical diameter slightlyless than the interior diameter of the socket. A portion of the ballinsert is formed to have an outer curved surface defining a cylinderhaving a maximum diameter less than the spherical diameter of the ballinsert and less than the diameter of the socket opening. The ball insertis introduced into the socket by aligning the cylindrical diameter withthe diameter of the opening and then placing the insert into the socket.The insert is thereafter rotated in the socket to juxtapose thespherical surface of the insert with the interior diameter of the socketto captively retain the insert.

Connection to the spinal rod is provided by way of a yoke that isengaged to the ball insert. In one embodiment, this engagement isaccomplished by a threaded bore in the ball insert and a mating threadedstem of the yoke. The ball insert is free to swivel in the fastenersocket and since the yoke is attached to the ball insert it is therebyalso free to move in a multi-axial manner. The yoke defines a channelbetween opposing arms of the yoke, with the channel configured to snuglyseat the rod therein. A sleeve is provided that fits about an upperportion of the head of the bone engaging fastener. This upper portionprovides a spherical surface to interface with a spherical lower cavityof the sleeve so that the sleeve may adapt a range of spherical anglesrelative to the bone engaging fastener as necessary to accommodate theposition of the spinal rod relative thereto. The sleeve includes in oneconfiguration opposing notches to receive and support the rod.

While the yoke does not itself support the spinal rod, it does support aset screw that is used to clamp the spinal rod to the notches in thesleeve. In one embodiment, the set screw is carried by a cap that fitsover and around the arms of the yoke. The set screw is configured toengage internal threads defined in the yoke arms so that as the setscrew is driven into the yoke a lower face of the screw contacts thespinal rod to drive it into the sleeve. In one embodiment, the set screwis supported within the cap so that the screw may rotate independentlyof the cap. The cap and the set screw may each define opposing groovesfor mutually carrying a retaining ring used to fix the set screw againstaxial movement relative to the cap. The retaining ring does permitrelative rotation so the set screw may be used to clamp the spinal rod.

The angular orientation of the yoke is adjusted relative to the boneengaging fastener to accommodate the position of the spinal rod relativeto the portion of the spine. In one feature of the invention, thisangular orientation is fixed by pressure engagement between the ballinsert and the spherical socket of the head of the bone fastener. Thus,in accordance with this feature, the present invention contemplates thatthe set screw not only operates to firmly clamp the spinal rod withinthe yoke and against the sleeve, it also generates an array of forcesthat press the ball insert into the spherical socket. In particular, asthe set screw is tightened within the threaded arms of the yoke, thepressure face of the set screw first contacts the spinal rod. As the setscrew is advanced further into the yoke, the pressure face clamps therod against the sleeve. At this point, the rod is generally firmly fixedto the yoke, although the yoke itself is not yet firmly fixed to thebone engaging fastener.

As the set screw is tightened further into the internal threads of theyoke arms, a reaction force is generated against the yoke itself, sincethe set screw cannot be driven any further into the rod of lower sleeve.This reaction force pulls the yoke upward, which in turn pulls the ballinsert upward due to the threaded engagement between the ball insert andthe threaded stem of the yoke. As the yoke and ball insert are pulledupward, the ball insert is pressed into the upper portion of thespherical socket of the head of the bone screw, thereby clamping theball insert relative to the bone screw. With the ball insert clamped,further tightening of the set screw pushes against the rod to drive thesleeve into firm locking engagement with the spherical outer surface ofthe head of the bone fastener.

In a particular aspect of the invention, an improved anchor device foranchoring an elongated rod to the spine is of the type comprising afastener having a bone engaging portion and a yoke having a rodreceiving channel that is supported by the fastener for swivel movementrelative thereto. In accordance with the improvement, the fastenerincludes a head defining a socket. An insert is captively retained inthe socket and is configured for swivel movement therein. A yokecomprises a portion attached to the insert so that the yoke is capableof movement with the insert. The yoke and insert are attached by aprocess of initially captively retaining the insert in the socket andthen attaching the yoke portion to the insert once captively retained.

One benefit of the present invention is that it provides for solidanchoring between a spinal rod and a bone engaging fastener at variablespherical angles. A further benefit is that a common clamping element isprovided to clamp the spinal rod and fix the angular position of theanchor device. Yet another benefit resides in one aspect of the anchordevice that reduces the overall prominence and profile of the componentsof the device. Other benefits of the invention can be discerned from thefollowing written description and accompanying figures.

DESCRIPTION OF THE FIGURES

FIG. 1 is a transverse view of a portion of a spine with a fixationsystem utilizing an elongated members engaged between successivevertebrae.

FIG. 2 is a side perspective view of an anchor device according to oneembodiment of the invention for use in the fixation system shown in FIG.1.

FIG. 3 is a top plan view of the anchor device shown in FIG. 2.

FIG. 4 is a side cross-sectional view of the anchor device shown in FIG.2.

FIG. 5 is a longitudinal cross-sectional view of the anchor deviceillustrated in FIG. 2 along the longitudinal axis of the elongatedmember.

FIG. 6 is a top plan view of a ball insert element of the anchor deviceshown in FIG. 2.

FIG. 7 is a side elevational view of the ball insert shown in FIG. 6.

FIGS. 8 a-8 f are side perspective views of a sequence of assembly ofthe components of the anchor device shown in FIG. 2.

FIG. 9 is a top perspective view of a sleeve component of the anchordevice shown in FIG. 2.

FIG. 10 is a side cross-sectional view of the sleeve shown in FIG. 9.

FIG. 11 is a longitudinal cross-sectional view of a fixture with holdingpins for holding the position of the ball insert relative to the socketduring engagement of the yoke.

FIG. 12 is a longitudinal cross-sectional view of the fixture withholding pins used to crimp or swage the threads of the yoke to fix theyoke to the ball insert.

FIG. 13 is a longitudinal elevational view of a cap with set screw ofthe anchor device of FIG. 2.

FIG. 14 is a top plan view of the cap shown in FIG. 13.

FIG. 15 is a cross-sectional view of the cap of FIG. 14 taken alongviewing line XV-XV.

FIG. 16 is a longitudinal cross-sectional view similar to FIG. 5 showingforces generated to lock the components of the anchor device of FIG. 2.

FIG. 17 is a side elevational view of a fastener inserter tool for usewith one embodiment of the anchor device of the present invention.

FIG. 18 is a longitudinal cross-sectional view of the fastener insertertool shown in FIG. 17 engaged to components of the anchor device shownin FIG. 2.

FIG. 19 is a longitudinal cross-sectional view of the lower end of a rodpersuader tool engaged to a partially assembled anchor device of FIG. 2.

FIG. 20 is a longitudinal cross-sectional view of an anchor deviceaccording to an alternative embodiment of the invention.

FIG. 21 is a longitudinal cross-sectional view of a screw inserter toolengaged to a partially assembled anchor device, such as the alternativeanchor device shown in FIG. 20.

FIG. 22 is a longitudinal side cross-sectional view of the lower end ofthe rod persuader tool engaged to a partially assembled modified anchordevice, such as the alternative device shown in FIG. 20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the invention is therebyintended. It is further understood that the present invention includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the invention aswould normally occur to one skilled in the art to which this inventionpertains.

The present invention contemplates a spinal fixation system, such as thesystem 10 depicted in FIG. 1. As is known in the art, the fixationsystem 10 spans between successive vertebrae of the spine. An elongatedmember, such as rod 12, extends along the length of the spine andprovides an anchor point for connecting each vertebra to the rod. Therod is typically contoured to approximate the normal curvature of thespine for the particular instrumented spinal segments. Anchor devices 15are provided for connecting the vertebral segments to the rod. Theseanchor devices may include hooks, bolts, screws or other means forengaging a vertebra. For the purposes of the present discussion, theanchor device 15 includes a bone engaging fastener 20 which is a bonescrew, as shown in FIG. 2. The bone screw 20 includes a threaded shank22 configured for threaded engagement within a portion of a vertebra. Ina specific example, the shank is configured for engagement within thepedicle of a vertebra.

The bone engaging fastener or screw 20 further includes a head 24 bywhich the screw, and ultimately the vertebra, is anchored to the spinalrod 12. In accordance with one feature of the present invention, thehead 24 defines a spherical socket 26 with a socket opening 28 facingthe rod, as shown in FIGS. 4-5. The bone screw 20 further defines acentral bore 30 intersecting the socket and extending part way into thethreaded shank 22. A transverse bore 32 extends through the head 24 andacross the socket, as best seen in FIG. 5. The function of the bores 30and 32 are discussed herein. The head 24 includes a spherical outersurface 34.

It can be appreciated from considering FIGS. 4-5 that the spherical head24 of the bone screw is more than simply hemispherical. In other words,the spherical socket 28 subtends a spherical angle of greater than 180°so that socket opening 28 is defined at a chord of the spherical socket.The planar diameter of the opening 28 at the chord is less than theinner diameter of the socket. In a specific embodiment, the sphericalhead subtends a spherical angle of about 24020 and the planar chordaldiameter of the socket opening 28 is about 90% the spherical diameter ofthe socket. It can thus be appreciated that a ball element of about thesame spherical diameter disposed within the socket will be retainedwithin the socket, unable to pass through the socket opening. It will beappreciated from the following discussion that a smaller planar chordaldiameter will reduce the range of angulation of the articulatingcomponents of the anchor device.

Of course, a ball element that is too large to pass through the opening28 cannot be readily inserted into the socket 26. The present inventionaddresses this matter by a ball insert element 40, illustrated in detailin FIGS. 6-7. The ball insert 40 defines a central threaded bore 42 thatis provided for connection to a yoke component 50, as described in moredetail herein. The ball insert is generally in the form of a truncatedsphere, whereby the outer surface 44 of the ball insert includes aspherical surface 45 that is sized to closely approximate the sphericalsocket 26, as shown in FIG. 5. Thus, spherical surface 45 defines anouter spherical diameter D₁, that is slightly less than the interiordiameter of the spherical socket 26, but greater than the diameter ofopening 28. As seen more particularly in FIG. 8 b, the ball insert 40 isfurther formed to have a cylindrical portion defined by curved surfaces46. The curved surfaces 46 of cylindrical portion define an outerdiameter D₂ about axis A as depicted in FIG. 8 b. Axis A in onearrangement is formed to be generally perpendicular to the axis of thecentral threaded bore 42. In accordance with one aspect of the inventionthe maximum diameter D₂ is slightly less than the planar chordaldiameter of socket opening 28 (FIG. 8 a) and defines an insert dimensionfor placing the ball insert 40 into the socket 26 as will be defined.While curved surfaces 46 are preferably formed to define a cylindricalinsert dimension D₂, it should be appreciated that other configurationsmay be considered, such as one or more flattened outer surfaces,provided that a maximum insert dimension such as diameter D₂ is formedless than the maximum dimension of the socket opening 28.

The benefit of this configuration for the ball insert 40 can be readilyappreciated upon consideration of the sequence in FIGS. 8 a-8 cdepicting insertion of the ball insert 40 into the socket 26 of the bonescrew 20. As shown in FIG. 8 b, the ball insert 40 is rotated at least90° so that the insert dimension D₂ with curved surfaces 46 is alignedto pass through planar chordal opening 28 and into the socket 26. Theinsert dimension D₂ is oriented so that axis A of ball insert 40 isessentially aligned along the axis of the bone screw. The depth of thesocket 26 is sufficient to fully receive the rotated ball insert 40 sothat the spherical surface 45 exposed in the view of FIG. 8 b is withinthe socket. Then, in the final step shown in FIG. 8 c, the ball insert40 is further rotated at least 90° so that the threaded bore 42 facesupward through the socket opening 28. In this position, the sphericalsurface 45 of the ball insert is juxtaposed with the interior of thespherical socket 26, as shown in FIG. 5, and the ball insert 40 iscaptively retained in the socket 26 for swivel movement therewithin.

The ball insert 40 is further provided along axis A as seen in FIG. 8 awith a transverse bore 48 that may be aligned with the transverse bore32 in the spherical head 24 of the bone screw, as shown in FIG. 5 andFIG. 8 a. As can be seen from the figures, the ball insert is truncatedat the top and bottom of the insert. However, the ball insert in thisarrangement is not symmetric—i.e., more of the top of the spherical ballis truncated than the bottom of the ball. Further, as a result of theformation of the curved cylindrical surfaces 46, the lower truncatedsurface has indentations 49 as illustrated in FIG. 7. When the ball 40is rotated as depicted in FIG. 8 b, the indentations 49 may be directedtoward the bottom of socket 26 and are not visible through the socketopening.

Returning to FIGS. 4-5, the anchor device 15 further includes a yoke 50having a threaded stem 52 configured to engage the threaded bore 42 inthe ball insert 40. The stem is provided with a shoulder 53 thatpreferably abuts the ball insert 40 when the stem 52 is fully threadedinto the bore 42 of the insert. The yoke 50 includes yoke arms 54 a, 54b that define a yoke channel 55 therebetween. The gap between the arms54 a, 54 b, and consequently the width of the channel, is sized toclosely fit the spinal rod 12, as best seen in FIG. 5. The arms 54 a, 54b define internal threads 56 at the upper open end of the yoke 50 forengaging a set screw 80, as described below. A bore 57 passes throughthe threaded stem 52 that is aligned with the bore 30 in the bone screwwhen the yoke is mounted on the ball insert.

As shown in FIGS. 8 d-8 e, a sleeve 60 is interposed between the yoke 50and the head 24 of the bone screw 20. As further shown in FIGS. 9 and10, the sleeve 60 defines a lower cavity 62 that has a sphericalconfiguration to substantially match the spherical outer surface 34 ofthe screw head 24. Sleeve 60 sits on the outer surface 34 for slidingmovement thereon, and serves as a clamping element for the rod 12relative to the yoke as will be described. The sleeve further defines anupper cavity 64 that generally parallels the outer surface of the yokearms 54 a, 54 b, as seen in FIG. 5. The upper face of the sleeve 60defines opposite rod grooves 66 sized to receive the spinal rod 12therein. The lower face of the sleeve defines opposite notches 68 thatare oriented 90° from the rod grooves 66. The notches 68 are arranged toalign with the transverse bores 32 and 48 when the anchor device isassembled. The notches and bores are sized to receive retaining pins 155(FIG. 11) as described in more detail herein. In a preferredarrangement, sleeve 60, is provided with opposing recessed surfaces 63that engage the arms 54 a, 54 b of the yoke 50 to key the sleeve 60 toyoke 50 in a manner that allows common swivel movement of the yoke 50and sleeve 60 relative to the screw head 24.

As depicted in FIG. 8 d-8 e, after the ball insert 40 is properlyaligned and captively retained as shown in FIG. 8 c, the yoke 50 mayengage the insert 40 to form an assembly therewith. In accordance withthe preferred manner of assembly of the anchor device 15, the threadedstem 52 of the yoke is extended through the sleeve 60 with the sleevekeying surfaces 63 aligned with the yoke arms 54 a, 54 b. The threadedstem 52 is then threaded into engagement with the threaded bore 42 ofthe ball insert. In order to achieve this threaded engagement it isnecessary to hold the ball insert 40 as the stem 52 of the yoke isthreaded into the bore 42. Thus, in one aspect of the invention, theball insert 40 is oriented within the spherical socket 26 so that thetransverse bores 48 in the insert are aligned with the transverse bores32 in the screw head. When the bores are aligned, pins 155 may be pushedtherethrough, taking care that the pins do not extend into the threadedbore 42, as illustrated in FIG. 11. Arms 157 of a forceps-like tool maybe used to introduce the pins into the bores.

With the pins 155 in position, the sleeve 60 may be placed over the headof the bone screw with the notches 68 aligned with the pins 155. Theyoke is then extended through the sleeve with the stem engaging thethreaded bore 42 of the ball insert. The pins 155 resist rotation of theball insert 40 as the stem is threaded into the bore. The yoke 50 isthreaded into the ball insert until the shoulder 53 contacts the upperface of the ball insert 40 as shown in FIGS. 4 and 5.

In an additional feature, the pins 155 may be used to crimp, swage ordeform the threads of the stem 52 of the yoke 50. Thus, the tool arms157 may be pressed toward each other so that the pins 155 contact thethreaded stem 52, as shown in FIG. 12. When the threads are distortedthe stem 52 of the yoke cannot back out or unthread from the ball insert40. Once the yoke and ball insert have been locked together, the pins155 can be removed. It is understood that this initial assembly of theanchor device, namely the steps shown in FIGS. 8 a-8 e, occur prior tointroduction of the anchor device 15 into the spine, preferably by thesupplier. It can also be appreciated that once the yoke 50 is lockedwith ball insert 40, the ball insert 40 is free to swivel within thefastener socket 26 allowing the yoke attached thereto to freely angulatein multiple directions. Since sleeve 60 is keyed to yoke 50 it likewisefreely slides on outer surface 34 of fastener head 24 as the yoke 50moves, until the anchor device components are locked in use.Furthermore, even though the ball insert 40 is free to swivel withinsocket 26, once the yoke 50 is attached the insert 40 remains captivelyretained since the insert 40 will not be able to move to a positionwhere its insert dimension L₁ is aligned with the socket opening 28.

Returning again to FIGS. 4-5, the assembly of the rod 12 to the fastener20 is shown. The rod 12 is initially placed between the arms of the yoke50 to rest on the rod grooves 66 of the sleeve 60. The yoke channel 55may then be closed, securing the rod within. In accordance with afurther feature of the invention, a cap 70 is fitted over the top of theyoke arms 54 a, 54 b. The cap 70 as further detailed in FIGS. 13-15,includes a generally cylindrical skirt 74 that fits snugly around thearms 54 a, 54 b to prevent the arms from splaying outward as set screw80 is threaded into the arms. The skirt 74 is preferably provided withdiametrically opposed flats 75 that correspond to the transverse openingof the yoke channel 55, as best seen in FIG. 8 f. The flats 75 definerod grooves 72 that align with, but do not contact, the rod 12 when itis situated within the yoke channel 55.

The cap 70 includes an upper boss 76 that defines an enlargedcircumferential interior groove 78. This groove is sized to receive aretaining ring or snap ring 90 therein, as seen in FIG. 5 and FIG. 15.The groove is axially enlarged or lengthened so that the snap ring 90may translate up and down within the boss 76 for reasons explainedbelow.

The set screw 80 is provided with a threaded stem 82 that is configuredto engage the internal threads 56 of the yoke arms 54 a, 54 b.Preferably the threaded engagement between set screw and yoke are in theform of buttress threads, as depicted in FIGS. 4-5. The buttress threadsminimize the outward force produced as the set screw is threaded intothe yoke. Thus, the use of buttress threads help minimize any splayingof the yoke arms that might otherwise occur when the set screw 80 isthreaded tightly into the yoke 50. In addition as shown in FIG. 15, thebottom of the set screw is recessed upwardly of the bottom of the skirt74 of cap 70. Thus, when cap 70 is placed over the arms 54 a, 54 b ofyoke 50, not only does the close fit of the skirt 74 relative theretoprevent splaying as noted, but skirt 74 also serves as a guide to alignthe threads 82 of set screw 80 into the threads 56 of the yoke 50,thereby also reducing the risk of disadvantageous cross-threading.

The set screw 80 includes a pressure face 83 that contacts and exerts asecuring force against the spinal rod 12. The pressure face 83 as wellas the rod surface may exhibit surface features intended to enhance thefixation between set screw and rod, as is known in the art. Inparticular, a surface roughness may be provided that becomes deformed orcold formed when the set screw is tightened onto the rod. This featurehelps prevent the rod from slipping axially (along its length) withinthe anchor device 15.

The set screw 80 defines a bore 84 therethrough. The upper portion 86 ofthe bore may be configured to receive a driving tool, such as with hexor TORX surfaces.

Like the cap 70, the set screw 80 defines a circumferential groove 88(FIG. 4) configured to receive the retaining ring 90 therein. However,unlike the cap groove 78, the groove 88 in the set screw is preferablysized to closely fit the snap ring. Thus, while the snap ring 90 is heldby the set screw, the snap ring is free to translate within theelongated cap groove 78. The elongated groove 78 is thus intended toallow the set screw 80 to fully engage the rod 12 while the cap 70essentially floats by virtue of the snap ring 90 translating withingroove 78. Thus, the cap 70 effectively exerts no force on the rod 12 oron the top surface of the yoke 50, even if some contact is made.

The set screw 80 generates the force that locks the ball insert 40within the spherical socket 26 at the desired angular orientation, andthat further locks the spinal rod 12 within the anchor device 15. Inparticular, once the anchor device 15 has been fully assembled about therod 12, as best seen in FIG. 16, the set screw 80 is tightened withinthe yoke 50. As the screw is tightened, it presses against the rod 12,clamping it between the pressure face 83 of the set screw and the rodgrooves 66 in the sleeve 60. As the set screw is driven further into theinternal threads 56 of the yoke 50, the set screw pushes the rod 12downwardly until the lower cavity 62 of the sleeve 60 is firmly engagedto the outer surface 34 of the head 24 of the bone screw generatinglocking force, F1. At this point the sleeve 60 and rod 12 can move nofurther toward the bone screw 20. Therefore, any further tightening ofthe set screw is reacted by the yoke itself. As the set screw is drivenfurther into the yoke internal threads (i.e., advancing toward the headof the bone screw) this reaction force pulls the yoke upward. While theyoke is pulled upward with continued rotation of the set screw, the stem52 of the yoke pulls the ball insert 40 upward, owing to the fixedengagement between the yoke stem and the ball insert. As the ball insertis pulled upward, it bears forcefully against the upper face of thespherical socket 26, with a force F2 clamping the socket wall betweenthe sleeve 60 and the ball insert 40 and thereby locking the ball insert40 and yoke 50 relative to fastener 20. Any tendency of the socket 26 toattempt to gap at the socket opening 28 is resisted by the sleeve 60that is already in firm engagement about the outer surface 34 of thescrew head.

It can thus be appreciated that the entire anchor device can beadjustably secured in a fixed relationship simply by rotation of the setscrew 80. As the set screw is threaded into the yoke threads it ensuressolid clamping of the bone screw head 24 between the lower cavity 62 ofthe sleeve 60 and the spherical surface 45 of the ball insert 40,regardless of the angular orientation of the yoke and rod relative tothe screw. The rod itself is firmly clamped between the set screw andthe lower sleeve. It can further be appreciated that the entire anchordevice may be tightened by simply tightening the set screw.

In use, the bone screw and sleeve assembly of FIG. 8 e is providedtogether with one or more suitably sized rods 12 and a cap 70 so that aspinal fixation system 10 may be implanted into a patient. The surgeonmay insert the bone screw assembly with a suitable screw inserter 140 asshown, for example, in FIGS. 17-18. The screw inserter 140 includes anouter sleeve 142 and an inner shaft 144 rotatably disposed within thesleeve. As shown in the view of FIG. 18, the end 146 of the outer sleeve142 is configured to contact the proximal upper surface of the sleeve60. The outer sleeve 142 is fixed to a handle 150, while the inner shaftis fastened to a tightening knob 152 that is rotatably supported on thehandle. The inner shaft 144 includes a pin end 148 that is sized toextend through the bore 57 in the yoke 50 and into the bore 30 at thebase of the spherical socket 26. The pin end 148 ensures co-axialalignment of the driving tool 140 and the bone screw threaded shank 22.The inner shaft further includes intermediate threads 149 axially offsetfrom the pin end 148. These threads 149 are arranged to engage theinternal threads 56 of the yoke arms 54 a, 54 b.

The threads 149 on the inner shaft 144 of the tool 140 operate similarto the set screw 80. Specifically, as the threads are driven into theinternal threads 56 of the yoke 50, the pin end 148 reacts against thebottom of the bore 30 in the bone screw to generate an upward force onthe yoke 50. As the yoke is pushed upward, it pulls the ball insert 40with it, thereby driving the insert into the spherical socket. When theinner shaft 144 has been fully tightened, the screw inserter tool 140,yoke 50, ball insert 40 and bone screw 20 form a rigid connection. Thehandle 150 of the outer sleeve 142 may then be used to drive the bonescrew into the vertebral bone, either manually or with the assistance ofan additional driving tool after a suitable hole has been drilled in thepedicle of a vertebra.

Once the bone screw 20 is threaded in position into the spine, the nextstep to completing the fixation system, such as system 10 shown in FIG.1, is to introduce the rod 12 into the yoke 50 of the anchor device 15.The rod may be contoured to match the normal curvature of the spine,either in lordosis or kyphosis depending upon the instrumented vertebrallevel. In some cases, the spine exhibits a lateral curvature, such asscoliosis, that is preferably corrected, at least partially, by thefixation system 10. Thus, in certain cases, the rod 12 itself may belaterally offset from the position of the bone screw engaged within theunderlying vertebra. In these cases, the variable angle capabilities ofthe anchor device of the present invention come into play.

To accomplish the introduction of the rod 12 into the yoke channel 55 ofthe yoke 50, a rod persuader tool 185 is provided, as shown in FIG. 19.The rod persuader tool 185 includes an outer tube 186 and an inner tube192 concentrically disposed within the outer tube for relative axialmovement. The outer tube 186 defines a rod notch 189 at its bottom end187. The inner tube 192 defines a slot 193 that forms legs 194 at thedistal end. The legs define an inner shoulder 195 that is configured tosuitably engage the partially assembled anchor device. The innershoulders 195 may engage a groove (not shown) in the outer surface 34 offastener socket 26. In another embodiment, the yoke 50 may be modifiedto have a groove (not shown) that may be engaged by the inner shoulders195. In either embodiment, the legs 194 are configured to partiallyencircle and firmly grasp the partially assembled anchor device, whilethe slot 193 accommodates the initial presence of the rod 12 within theyoke channel 55. A guide pin 190 spans the diameter of the outer tube186 and fits within the slot 193 to control the relative axial movementbetween the outer tube 186 and the inner tube 192. A suitable mechanismis provided to move the outer tube 186 downward axially relative toinner tube 192. As the outer tube 186 moves downward, it forces the rod12 into the yoke channel 55 by lower notch 189 and into the rod groove66 of the sleeve 60.

With the rod 12 suitably placed into the yoke 50, the spinal fixationdevice 10 may then be completed. Cap 70 as shown in FIG. 8 f is thenassembled to the yoke 50, as described above with reference to FIGS.4-5, to lock the rod 12 relative to the yoke 50 and the yoke 50 relativeto the bone fastener 20. It should be appreciated that the spinalfixation device 10 as particularly described herein has the advantage ofestablishing a low profile, since the outer surface of the screw head 24may be driven down relatively deeply into the pedicle of the vertebra,while still maintaining swivel movement of the yoke 50 until the setscrew 80 is tightened. Furthermore, the relatively large surface area ofspherical surface 45 of the ball insert 40 tightly pressed against theinterior surface of the screw socket 26 provides for a very rigidconstruct for locking the polyaxial motion of the yoke 50 relative tothe screw 20.

Having described one particular embodiment of the anchor device for usein spinal fixation device 10, it should be understood that alternativeembodiments are also contemplated. One alternative as illustrated inFIG. 20, also uses the captively retained ball insert 40 describedabove. An alternative anchor device 100 shown in FIG. 20 includes a boneengaging fastener 102 with a spherical head 103. The head defines aspherical socket 105 like the bone screw 20 described above. The ballinsert 40, lower sleeve 60, upper sleeve 70 and set screw 80 may beconstructed as described above. The yoke 110 includes a threaded stem111 and a shoulder 115 for threaded engagement with the ball insert.However, unlike the previously described yoke 50, the yoke 115 includesan internal cavity 113 extending from the distal end 117. This cavitycorresponds to a dimple 107 formed in the base of the spherical socket105.

In accordance with this embodiment, a retention ball 120 is seatedwithin the dimple and residing within the cavity 113. A spring 121 isdisposed within the cavity to exert a relatively slight force againstthe ball 120. The ball and dimple serve as a releasable detent tomaintain a proper orientation between the ball insert 40 and the screwhead 103 for ease of screw insertion. The spring maintains pressure onthe seating ball 120 and also exerts an upward force on the ball insert40 to help engage the insert within the spherical cavity 113 of the bonescrew head. The fixation of the anchor device 100 otherwise proceeds asoutlined above by tightening the set screw 80.

A suitable insertion tool 200 for inserting device 100 into a vertebrais shown in FIG. 21. Tool 200 has an inner tube 202 with shoulders 204partially configured to engage a groove 206 formed in the outer surfaceof yoke 110. Tool 200 further has a driver element 208 that isconfigured to fit within the channel of yoke 110 to thread the device100 into a vertebra.

A rod persuader tool 220 for particular use with device 100 is shown inFIG. 22. Tool 220 is similar to the rod persuader 185 having an outertube 186 axially movable with respect to an inner tube 192. Theshoulders 204 are configured to engage yoke groove 206 and hold thedevice 100 while an elongated rod 12 is pushed into the yoke 110 of thedevice 100. The rod is pushed by notch 189 upon downward movement ofouter tube 186. Completion of the spinal fixation system takes place byassembly of the cap 70 over the yoke 110 and tightening of the set screw80 against the rod 12 as described above.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. It is understood thatonly the preferred embodiments have been presented and that all changes,modifications and further applications that come within the spirit ofthe invention are desired to be protected.

1. A spinal fixation system comprising: an elongated rod sized to extend between at least two vertebrae; a fastener having a bone engaging portion and a head, said head defining a socket having a socket opening, said socket having an interior dimension and said socket opening having a planar dimension less than said interior dimension; an insert situated in said socket and configured for swivel movement therewithin relative to said bone engaging portion, said insert being of fixed size and configuration and including an outer surface comprising a first portion having a first dimension that is greater than said planar dimension of said socket opening and less than said interior dimension of said socket and a second portion having a maximum second dimension that is less than said planar dimension of said socket opening; a yoke defining a yoke channel for receiving said rod therein and including a portion configured to be secured to said insert at said first portion when said insert is within said socket for joint swivel movement of said yoke with said insert; and a clamping structure for clamping said elongated rod within said yoke while locking said yoke relative to said fastener.
 2. The spinal fixation system of claim 1, wherein: said socket is a spherical socket and said interior dimension is a spherical diameter; said socket opening is circular and said planar dimension is a chordal diameter; and said insert is a truncated sphere, said first dimension being a spherical diameter.
 3. The spinal fixation system of claim 2, wherein said truncated sphere further includes a cylindrical portion defined by at least one curved surface, the maximum diameter of said cylindrical portion defining said second dimension of said insert.
 4. The spinal fixation system of claim 2, wherein: said truncated sphere has a bore extending therethrough generally perpendicular to said second portion; and said yoke includes a stem extending into said bore in securement with said truncated sphere.
 5. The spinal fixation system of claim 4, wherein: said bore in said insert is internally threaded; and said stem of said yoke includes mating external threads.
 6. The spinal fixation system of claim 5, wherein said stem further defines a shoulder between said yoke channel and said stem for contacting said insert when said stem is threaded into said bore of said insert.
 7. The spinal fixation system of claim 1, wherein: said yoke channel defines internal threads; and said clamping structure includes a set screw configured for threaded engagement with said internal threads of said yoke channel.
 8. The spinal fixation system of claim 7, wherein said set screw includes buttress threads.
 9. The spinal fixation system of claim 7, wherein: said yoke includes opposite arms defining said yoke channel therebetween and said internal threads; and said clamping structure includes a cap having a skirt configured to surround said arms of said yoke and a central boss configured to rotatably support said set screw.
 10. The spinal fixation system of claim 9, wherein: said yoke arms define a cylindrical outer surface; and said skirt of said cap includes a cylindrical portion corresponding to cylindrical outer surface of said yoke arms and a generally flat portion corresponding to said yoke channel.
 11. The spinal fixation system of claim 9, wherein: said central boss defines an interior circumferential groove; said set screw defines an exterior circumferential groove; and said cap includes a retaining ring spanning between said central boss and said set screw and disposed within said corresponding circumferential grooves.
 12. The spinal fixation system of claim 11, wherein said interior circumferential groove of said central boss is elongated to permit translation of said retaining ring along said groove.
 13. The spinal fixation system of claim 11, wherein said retaining ring is a snap ring.
 14. The spinal fixation system of claim 1, wherein said clamping structure includes a sleeve extending at least partially around said yoke and supported by said head of said fastener, said sleeve defining a groove along an edge thereof sized to receive and support said elongated rod therein.
 15. The spinal fixation system of claim 1, wherein said fastener is a bone screw and said bone engaging portion includes a threaded shank.
 16. An anchor device for anchoring an elongated rod to the spine, comprising: a fastener having a bone engaging portion and a head, said head defining a socket; an insert of fixed size and configuration including an outer surface having a spherical portion and a non-spherical portion, said insert being captively retained within said socket and defining a bore extending into said non-spherical portion, said insert configured to swivel within said socket relative to said bone engaging portion; a yoke defining a yoke channel for receiving the elongated rod therein and including a stem engaging said bore in said insert while said insert is within said socket; and a clamping element configured for cooperatively clamping the elongated rod within said yoke while locking said yoke relative to said fastener.
 17. The anchor device of claim 16, wherein: said socket is a spherical socket defining an interior spherical diameter and having a socket opening defining a chordal diameter; and said insert is a truncated sphere having a spherical diameter that is less than said interior spherical diameter but greater than said chordal diameter, and further having a dimension transverse to the axis of said bore that is less than said chordal diameter of said socket opening.
 18. The anchor device of claim 16, wherein: said bore in said insert is internally threaded; and said stem of said yoke includes mating external threads.
 19. The anchor device of claim 18, wherein said stem further defines a shoulder between said yoke channel and said stem for contacting said insert when said stem is threaded into said bore of said insert.
 20. The anchor device of claim 16, wherein: said yoke channel defines internal threads; and said clamping element includes a set screw configured for threaded engagement with said internal threads of said yoke channel.
 21. The anchor device of claim 20, wherein said set screw includes buttress threads.
 22. The anchor device of claim 20, wherein: said yoke includes opposite arms defining said yoke channel therebetween and said internal threads; and said clamping element includes a cap an upper sleeve having a skirt configured to surround said arms of said yoke and a central boss configured to rotatably support said set screw.
 23. The anchor device of claim 22, wherein: said yoke arms define a cylindrical outer surface; and said skirt of said cap includes a cylindrical portion corresponding to cylindrical outer surface of said yoke arms and a generally flat portion corresponding to said yoke channel.
 24. The anchor device of claim 22, wherein: said central boss defines an interior circumferential groove; said set screw defines an exterior circumferential groove; and said cap includes a retaining ring spanning between said central boss and said set screw and disposed within said corresponding circumferential grooves.
 25. The anchor device of claim 24, wherein said interior circumferential groove of said central boss is elongated to permit translation of said retaining ring along said groove.
 26. The anchor device of claim 24, wherein said retaining ring is a snap ring.
 27. The anchor device of claim 16, wherein said clamping element includes a sleeve extending at least partially around said yoke and supported by said head of said fastener, said sleeve defining a groove along an edge thereof sized to receive and support the elongated rod therein.
 28. The anchor device of claim 16, wherein said fastener is a bone screw and said bone engaging portion includes a threaded shank.
 29. The anchor device of claim 28, wherein said clamping element further includes a set screw threadedly engaged within said yoke channel for bearing against the rod disposed within said channel.
 30. An anchor device for anchoring an elongated rod to the spine, comprising: a fastener having a bone engaging portion and a head, said head defining a socket; an insert within said socket, said insert being configured to swivel in said socket relative to said bone engaging portion and including an outer surface comprising a first portion having a first dimension allowing said insert to be received in said socket and a second portion having a second dimension causing said insert to be captively retained in said socket; a yoke defining at one end a yoke channel for receiving said elongated rod therein and at an opposite end a support portion affixed to said insert at said first portion while captively retained in said socket, said yoke with said insert being capable of swiveling within said socket; a securing member configured to operatively engage said yoke at said channel end to apply a force on said rod when said rod is received within said yoke channel; and a clamping element supported exteriorly of said fastener by said head of said fastener and having a surface to receive said elongated rod, said clamping element securing said rod to said yoke upon application of force by said securing member on said rod.
 31. The anchor device of claim 30, wherein said socket has an interior dimension and a socket opening has a planar dimension less than said interior dimension.
 32. The anchor device of claim 31, wherein: said socket is a spherical socket and said interior dimension is a spherical diameter; said socket opening is circular and said planar dimension is a chordal diameter; and said insert is a truncated sphere, said first dimension being a spherical diameter.
 33. The anchor device of claim 32, wherein said truncated sphere further includes a cylindrical portion defined by at least one curved surface, the maximum diameter of said cylindrical portion defining said second dimension of said insert.
 34. The anchor device of claim 33, wherein: said truncated sphere has a bore extending therethrough generally perpendicular to said second portion; and said yoke includes a stem extending into said bore in securement with said truncated sphere.
 35. The anchor device of claim 30, wherein: said yoke channel defines internal threads; and said clamping structure includes a set screw configured for threaded engagement with said internal threads of said yoke channel.
 36. The anchor device of claim 35, wherein said set screw includes buttress threads.
 37. The anchor device of claim 30, wherein said clamping structure includes a sleeve extending at least partially around said yoke and supported by said head of said fastener, said lower sleeve defining a groove along an edge thereof sized to receive and support said elongated rod therein.
 38. An anchor device for anchoring an elongated rod to the spine, comprising: a fastener having a bone engaging portion and a head, said head defining a socket: an insert within said socket, said insert being configured to swivel in said socket relative to said bone engaging portion and including an outer surface comprising a first portion having a first dimension allowing said insert to be received in said socket and a second portion having a second dimension causing said insert to be captively retained in said socket; a yoke defining at one end a yoke channel for receiving said elongated rod therein and at an opposite end a support portion affixed to said insert at said first portion while captively retained in said socket, said yoke with said insert being capable of swiveling within said socket; a securing member configured to operatively engage said yoke at said channel end, said securing member configured to apply a force to said rod; a clamping element supported by said fastener and including an upper surface to receive said elongated rod while in said yoke channel, and a lower surface supported by an exterior surface of said fastener head, said clamping element upon application of a force on said rod clamping said rod relative to said yoke and clamping said fastener head between said clamping element and said insert to thereby lock said insert and yoke relative to said fastener.
 39. The anchor device of claim 38, wherein said clamping element includes a sleeve supported by said head of said fastener and at least partially surrounding said yoke, said sleeve having an upper groove for receiving and supporting said elongated rod thereon, and a lower interior surface contacting an outer surface of said head of said fastener.
 40. In an anchor device for anchoring an elongated rod to the spine of the type comprising a fastener having a bone engaging portion and a yoke having a rod receiving channel and supported by said fastener for swiveling movement, the improvement comprising: said fastener including a head defining a socket; an insert including an outer surface having a spherical portion and a non-spherical portion, said insert captively retained in said socket and configured for swiveling therein relative to said bone engaging portion; said yoke including an integral portion attached to said insert at said non-spherical portion; and said yoke and said insert attached by a process of initially captively retaining said insert in said socket and then attaching said portion of said yoke to said captively retained insert for swiveling jointly therewith.
 41. The improvement of claim 40, wherein said socket is a spherical socket having a spherical diameter and an opening, said opening having a planar chordal diameter less than said spherical diameter.
 42. The improvement of claim 41, wherein said insert spherical portion has a first dimension greater than said chordal diameter and less than said spherical diameter and said insert non-spherical portion has a second dimension less than said chordal diameter.
 43. The improvement of claim 42, wherein said second dimension of said insert non-spherical portion defines an insert direction and said first dimension of said insert spherical portion defines a captive dimension, and wherein the process of captively retaining said insert in said socket comprises the steps of placing said insert into said socket through said socket opening along said insert direction of said non-spherical portion and rotating said insert therewithin.
 44. The improvement of claim 41, wherein said insert is a truncated sphere having a spherical diameter less than the spherical diameter of said socket.
 45. The improvement of claim 44, wherein said truncated sphere further includes a cylindrical portion defined by at least one curved surface, the maximum diameter of said cylindrical portion being less than said chordal diameter.
 46. An anchor device for anchoring an elongated rod to the spine comprising: a fastener having a bone engaging portion and a head, said head defining a socket having a socket opening, said socket having an interior dimension and said socket opening having a planar dimension less than said interior dimension; an insert situated in said socket and configured for swivel movement therewithin relative to said bone engaging portion, said insert including an outer surface comprising a first portion having a first dimension that is greater than said planar dimension of said socket opening and less than said interior dimension of said socket and a second portion having a second dimension that is less said planar dimension of said socket opening; and a yoke defining a yoke channel for receiving said rod therein and including an integral portion configured to be secured to said insert at said first portion when said insert is within said socket for swiveling jointly therewith.
 47. An anchor device for anchoring an elongated rod to the spine comprising: a fastener having a bone engaging portion and a head, said head defining a socket having a socket opening, said socket having an interior dimension and said socket opening having a planar dimension less than said interior dimension; an insert situated in said socket and configured for swivel movement therewithin relative to said bone engaging portion, said insert being of fixed size and configuration and including an outer surface comprising a first portion having a first dimension that is greater than said planar dimension of said socket opening and less than said interior dimension of said socket and a second portion having a maximum second dimension that is less said planar dimension of said socket opening; and a yoke supported by said insert at said first portion and including a yoke channel for receiving said rod therein.
 48. The anchor device of claim 47, further including a clamping element supported by the head of said fastener and extending at least partially around said yoke. 